Charge balancing system

ABSTRACT

The problem of battery failure due to failure of one cell in a rechargeable battery, and the related problem of inefficient use of a battery over its dynamic range due to differences between the performance of cells in a battery, are addressed by providing one or more capacitors selectively coupled to the various cells of the battery. The selective and repetitive coupling of capacitors to the cells permits balancing of charge among the cells. This minimizes the risk that any one cell would suffer catastrophic failure due to being fully charged or discharged prior to the other cells in the battery. This also permits making use of the battery over nearly all of its dynamic range. In this way, battery life is maximized.

[0001] This application claims priority from U.S. application Ser. No.60/178,887, filed Jan. 28, 2000, which application is incorporatedherein by reference to the extent permitted by law.

[0002] The invention relates generally to the problem of limited servicelife in multi-cell batteries, and relates more specifically toapproaches for balancing of charge among cells.

BACKGROUND

[0003] Many users want appliances to be portable, even if the applianceshave historically been fixed in position due to the need for power andother connections. For many appliances, it is a straightforward matterto make the appliance portable by shrinking it (making it smaller) andby powering it with battery power rather than line power. For those whoare given the task of making an appliance portable, however, it soonbecomes clear that there are many problems to be solved before theappliance will fulfill user expectations. For example, in a personalcomputer or personal digital assistant or wireless telephone, the usernaturally wants long battery life as well as minimal weight. It isfurther desirable that battery life be long in two distinct senses—thebattery should last a long time in use, between rechargings; and thebattery should survive a large number of charge/discharge cycles. Wheredisposable batteries are used, the former is the only concern, but manyusers prefer to use rechargeable batteries for which both senses of theterm “long life” are relevant.

[0004] One way to try to maximize battery life is by selecting a batterytechnology appropriately for the task. Some early personal computersused sealed lead-acid batteries. For many years the prevailing batterytechnology was nickel-cadmium More recent choices have includednickel-metal-hydride and lithium ion technology. From the user's pointof view, it is desirable to select a technology that offers a highspecific capacity (capacity versus weight) so as to reduce the weight ofthe appliance.

[0005] These “long life” goals are not easy to meet. In any battery(defined as a plurality of cells in series) there is the problem thatthe cells are not perfectly identical, even though the batterymanufacturer will try as hard as possible to match the cells with eachother. Because the cells are not identical, they do not have identicalcapacities. If a battery is allowed to run down nearly to falldischarge, one cell will reach full discharge before its neighbors, andwill suffer chemical degradation if current continues to be forcedthrough it (by its neighbors) during further discharge. This so-called“reverse charging” problem can ruin a cell in a very short time,depending on its particular technology.

[0006] Yet another problem arises when the battery is to be recharged.Suppose the cells were to have identical charging qualities, that is,that a particular charge forced through each cell would give rise toprecisely the same extent of recharging. Even with such cells, theproblem is that they might not have begun with the same level of charge(due to previous use, for example). As such, if they areseries-connected and given a particular amount of charging current, theywould not reach full charge simultaneously. As a general matter, onecell would reach full charge sooner than its neighbors. Such a cellfaces the problem of having to find some way to dissipate excess energy,typically through radiated heat, during the time when the remainingcells continue to be charged. This “overcharging” problem can damage acell through any of a number of mechanisms (depending on the celltechnology) including the conversion of water to steam.

[0007] In real life, cells do not have identical charging qualities, sothis problem (of one cell reaching fill charge before its neighbors do)is exacerbated.

[0008] While some cell technologies are fairly accommodating of problemslike reverse charge and overcharging, other technologies are quitevulnerable to them. Even the most accommodating technologies, however,will sooner or later lead to a “bad cell” in a battery, whether due toreverse charging, overcharging, or some other failure mode. When a cellgoes bad, typically the battery must be removed from service andreplaced by a new battery.

[0009] One approach for protecting against reverse charging is to stopusing the battery (and then to recharge it) well before any of the cellscould possibly have reached full discharge. This approach leads to arather poor ratio of weight to service life between rechargings.

[0010] One approach for protecting against overcharging is to stopcharging the battery (and then to start using it it) well before any ofthe cells could possibly have reached full charge. This approach, too,leads to a rather poor ratio of weight to service life betweenrechargings.

[0011] Experience shows that the life of a particular cell is nottypified by perfect and identical performance for many cycles, followedby catastrophic failure as a “bad cell.” Instead, the life of aparticular cell is typified by slight degradations in performance overits life, followed by catastrophic failure. (Of course, most cells neveractually reach catastrophic failure because they are part of a batterythat is removed for service because some other cell in the battery hasreached catastrophic failure.) Slight degradations include small lossesin charging efficiency, and small increases in the internal resistanceof the cell giving rise to waste heat during discharge and charging.Such degradations would be only a small problem if they affected allcells equally, but of course experience with actual cells shows thatthey are not uniform across all cells.

[0012] Such slight degradations, if not provided for in the batterysystem design, set the stage for imbalances which will accelerate theprogress toward eventual catastrophic loss of a single cell.

[0013] This accelerated progress toward failure may be described as a“race to the bottom,” in which the life of the battery is essentiallydefined by (and limited by) the life of the worst cell.

[0014] It would thus be extremely desirable if a system could be devisedwhich would minimize the extent to which non-identicality of cells in abattery leads to premature catastrophic failure of any one cell. Itwould also be extremely desirable if such a system could likewise permituse of a battery over a wide dynamic range, e.g. from nearly full chargeto nearly full discharge, thus maximizing battery life betweenchargings, all without taking unnecessary risk of reverse charging orovercharging.

[0015] One approach to this problem may be seen in PCT publication no.99-21241, published Apr. 29, 1999 and entitled Improved voltaic pilewith charge equalizing system. The arrangement in the PCT publication,however, has a relatively high component count. It would be desirable todevise an approach having a smaller component count.

SUMMARY OF THE INVENTION

[0016] The problem of battery failure due to failure of one cell in arechargeable battery, and the related problem of inefficient use of abattery over its dynamic range due to differences between theperformance of cells in a battery, are addressed by providing one ormore capacitors selectively coupled to the various cells of the battery.The selective and repetitive coupling of capacitors to the cells permitsbalancing of charge among the cells. This minimizes the risk that anyone cell would suffer catastrophic failure due to being fully charged ordischarged prior to the other cells in the battery. This also permitsmaking use of the battery over nearly all of its dynamic range. In thisway, battery life is maximized.

DESCRIPTION OF THE DRAWING

[0017] The invention will be described with respect to a drawing inseveral figures, of which:

[0018]FIG. 1 shows a prior-art charge balancing system using dissipativebalancing;

[0019]FIG. 2 shows a charge balancing system according to the invention,using nearly the same number of capacitors as cells; and

[0020]FIG. 3 shows a charge balancing system according to the invention,using a single capacitor.

[0021] Where possible, like elements have been denoted among the figuresusing like reference numerals.

DETAILED DESCRIPTION

[0022] To understand and appreciate the invention fully, it is helpfulto discuss one prior-art approach for charge balancing in a battery.This discussion identifies terminology which is helpful in describingembodiments of the invention. FIG. 1 shows a prior-art charge balancingsystem 11 using dissipative balancing. A number of cells 22, 23, 24, and25 is disposed in series to define a battery between terminals 20 and21. Terminals 20, 21 connect to a load (typically an appliance such as acomputer or PDA or telephone) and may be connected to a source ofcharging current for recharging.

[0023] Resistors 32, 33, 34, and 35 are associated respectively with thecells 22, 23, 24, and 25 and with switches 42, 43, 44 and 45. Controllogic 56 causes the switches to be selectively opened and closed. Eachswitch, when closed, drains a predictable amount of current from itsrespective cell through its respective resistor.

[0024] During charging, it is possible to use the system to protect acell that is charging faster than the other cells from overcharging. Theswitch associated with that cell can be closed, thus discharging thecell slightly (or reducing its rate of charging slightly) so as to bringthe cell more nearly into balance with its neighbors.

[0025] A similar approach may be followed during discharge. If one cellis seen to progress toward discharge less quickly than others, then theswitch for that cell can be turned on, draining that cell a littlefaster and tending to bring it more into balance with its neighbors.

[0026] The control logic 56 thus ideally receives inputs from monitoringcircuitry, not shown, which permits identifying cells which are chargingor discharging faster or slower than their neighbors.

[0027] The system 11 of FIG. 1 has the advantage that it will tendtoward balanced charge among cells in a battery, and thus will maximizebattery life and postpone failure of any one cell. It has the drawback,however, of wasting energy. Any energy dissipated in a resistor 32, 33,34, 35 is energy which had to be stored in a heavy battery and yet whichdoes not get used in the appliance. To obtain a particular battery lifebetween rechargings, then, requires a heavier battery than would be usedif the dissipative resistor system were not used.

[0028] The system 11 of FIG. 1 also requires sensors such as voltage ortemperature sensors on each of the cells, to provide inputs to thecontrol logic 56. The sensors must be accurate; if they provideincorrect information the cells will not be handled properly. Thealgorithm carried out by control logic 56 is complicated. It ispossible, through inadvertence, to make mistakes in such algorithms,such as failure to take account of boundary conditions. The controllogic risks getting into some internal state which freezes up the systemand requires a reset (and thus may require a watchdog timer).Rechargeable battery technologies differ in their response to abuse, butsome technologies can risk damage (e.g. thermal runaway) upon failure ofthe control logic. Simple systems are often less at risk of malfunction.

[0029] A crude but limiting case in the programming of the system ofFIG. 1 may be seen in a system which employs the resistors and switchesonly after the battery has “run down” and is ready for recharging. Priorto recharging, the switches are all turned on, and thus each celldischarges fully through its resistor. In this way, all of the cellsstart from the same initial condition when charging begins. Thisapproach has many drawbacks, not the least of which is that some celltechnologies work best if no cell is ever discharged all the way.

[0030] Turning now to FIG. 2, what is shown is a charge balancing system12 according to the invention. Terminals 20, 21 and cells 22, 23,24, 25are as before. Switches 52, 53, 54, 55 serve as level shifters anddrivers. Capacitors 62, 63, 64 are as shown. An oscillator 57 causes theswitches to move in synchrony. With one state of the oscillator, each ofthe switches connects its topmost terminal to the rightward terminal andthus to the capacitor array. With another state of the oscillator, eachof the switches connects its bottom-most terminal to the rightwardterminal and thus to the capacitor array.

[0031] In this way, the state of affairs is as follows.

[0032] A. In one state, capacitors 62, 63, and 64 are paralleled withcells 22, 23, 24 respectively.

[0033] B. In the other state, capacitors 62, 63, and 64 are paralleledwith cells 23, 24, 25 respectively.

[0034] Let us now analyze the system in the (idealized) case where allthe cells behave identically to each other, namely, in which each cellalways has the same voltage as the other cells, during charge cycles anddischarge cycles. In such a case, initially a brief current will flow sothat each of the capacitors is charged to the same potential, andthereafter no current will flow into or out of the capacitors. In suchan idealized case, the function and performance of the system 12 will bethe same regardless of whether the switches and capacitors are present.

[0035] The idealized case, however, is not realistic. In a realisticcase, it develops from time to time that one of the cells will have avoltage higher or lower than another of the cells. For example, let ussuppose that cell 22 has a higher charge than cells 23, 24, 25.Eventually capacitor 62 is placed in parallel with cell 22, and ischarged to its level. Later, capacitor 62 is placed in parallel withcell 23. Because it is at a higher voltage, it discharges into the cell23. After some number of cycles, the cells 22 and 23 will arrive at thesame voltage.

[0036] Likewise any imbalance between cells 23 and 24 is graduallyremedied by the paralleled connections of the capacitor 63 from time totime between the two cells.

[0037] Likewise any imbalance between cells 24 and 25 is graduallyremedied by the paralleled connections of the capacitor 64 from time totime between the two cells.

[0038] In this way, charge is passed upwards and downwards amongadjacent cells of the battery so as to keep the cells balanced. Duringdischarge, no one cell is likely to reach full discharge before theothers. During charge, no one cell is likely to reach full charge beforethe others. This postpones the day when the battery would have to betaken out of service because of poor performance of any one cell.

[0039] In the discussion of the system 11 of FIG. 1, it was mentionedthat many of the circuit elements would have to be quite accurate forthe system to work well. It is instructive to consider how tolerant thesystem 12 of FIG. 2 may be of variations in the components used.

[0040] The capacitors 62, 63, 64 need not be identical or even close toidentical. The switches 52, 53, 54, 55 do need to be fairly closelymatched, at least in terms of their voltage drops. MOSFETs are preferredsince they minimize the voltage drops and thus minimize the extent towhich one cell could deviate in voltage from a neighboring cell.

[0041] The oscillator 57 is not critical. It need not oscillate at anyparticular frequency, nor is there a requirement that its duty cycle be50-50.

[0042] The conductors between the cells and the switches, and betweenthe switches and the capacitors, need not be large in current carryingcapacity. This is because the currents needed to balance the cells'voltages need not be large, since the voltages are unlikely to deviategreatly from each other due to the continuous equalizing that would havepreviously taken place.

[0043] The switches must be bidirectional, since at any moment a cellmight need to be charged or discharged. The switches must bebreak-before-make, that is, they must not permit the cells to beshorted. The sizing of the capacitors is not critical. If the capacitorsare too small, then there is the risk that the charge-redistributionsystem would not be able to keep up with the performance differences ofthe cells. If the capacitors are too large, then there is the problemthat space and weight are taken up by the too-large capacitors.

[0044] Stated differently, the apparatus may thus be described asfollows. A battery comprises a plurality of n rechargeable cells 22, 23,24, 25 in series-adjacent connections, each cell having a positiveterminal 20 and a negative terminal 21. Also provided are a plurality ofswitches 52, 53, 54, 55, each corresponding with one of the cells, eachswitch having first, second, and third terminals, each switch switchablebetween a first position in which its first and third terminals areconnected therebetween and a second position in which its second andthird terminals are connected therebetween. The first terminal of eachswitch is connected with the positive terminal of the correspondingcell, and the second terminal of each switch is connected with thenegative terminal of the corresponding cell. N-I capacitors 62, 63, 64are provided, each connected between third terminals of adjacentswitches. Control circuitry 57 such as an oscillator is provided,causing all of the switches to be switched to the first position, andcausing all of the switches to be switched to the second position.

[0045] A method of operation of such cells, switches, and capacitors maybe described as repeatedly performing the steps in sequence of: causingall of said switches to be switched to said first position; and causingall of said switches to be switched to said second position.

[0046] Yet another way to describe a method of operation for use with abattery comprising a plurality of n cells in series adjacentconnections, each cell having a positive terminal and a negativeterminal; and a plurality of n-1 capacitors in series adjacentconnections, is to repeatedly perform the steps in sequence of:connecting the capacitors each in parallel with respective ones of thefirst through n-1th cells; and connecting the capacitors each inparallel with respective ones fo the second through n-th cells. Thismore general statement, fully consistent with the invention, leaves openthe particular manner in which the connections might be made.

[0047]FIG. 3 shows a charge balancing system 13 according to theinvention, using a single capacitor, in an embodiment thus differingfrom that described above in connection with FIG. 2. Battery 20, 22, 23,24, 25, 21 is as described above. A two-pole multiplexer 58 is provided,by definition containing within it a first single-pole multiplexer 66and a second single-pole multiplexer 67. These multiplexers selectivelyconnect capacitor 65 to various of the cells 22, 23, 24, 25. Any cellhaving a voltage higher than that of the others will tend to charge upthe capacitor 65, which will in turn discharge itself into one or moreof the cells that had a lower voltage. In this way, the cells are keptmore or less in balance with each other. The multiplexer 58 does thisunder control of an oscillator/counter 59 which causes the multiplexerto move among its positions.

[0048] In a simple case, the oscillator/counter 59 steps seriatimthrough its positions. It will be appreciated, however, that nothingabout the inventions requires the multiplexer to step seriatim. Indeedit might be considered desirable to control the multiplexer so that itfollows a pseudo-random sequence encompassing all possible two-cellsequences, so that from time to time each cell has an opportunity topass along charge to each of the other cells.

[0049] Describing the apparatus in a different way, what is provided inthis embodiment is a battery comprising a plurality of n rechargeablecells 22,23, 24, 25 in series adjacent connections, each cell having apositive terminal 20 and a negative terminal 21. Also provided are firstand second multiplexers 66, 67 each having a plurality of n firstterminals and having a second terminal, the first terminals of the firstmultiplexer connected with the positive terminals of the cells, thefirst terminals of the second multiplexer connected with the negativeterminals of the cells, each multiplexer switchable between n positionsin which an n-th first terminal is connected with the second terminal.Also provided is a capacitor 65 connected between second terminals ofthe first and second multiplexers 66, 67. Also provided is controlcircuitry 59 causing the first and second multiplexers 66, 67 to beswitched to corresponding positions among the n positions.

[0050] All that is required is that the two multiplexers 66, 67 be movedin more or less synchrony, so that each cell is paralleled with thecapacitor 65 for a nonzero interval of time. The intervals need not beidentical, though it is probably simplest to make the intervalsidentical. Each multiplexer must be break-before-make so that it willnot short out a cell. The capacitor 65 need not be of any exact value,but merely needs to be large enough to keep up with whatever possibleimbalance of charge turns out to be necessary to correct. If capacitor65 is too large, this does not degrade system performance in any way butmerely takes up space and adds weight. The multiplexers must bebidirectional.

[0051] It is also possible to characterize a method to be performed withthe cells, multiplexers, and capacitor that have been described, namely:repeatedly performing the step of causing the first and secondmultiplexers to switch both to a corresponding new position.

[0052] A somewhat more general way to characterize the method, withoutdeparting in any way from the invention, is that with a batterycomprising a plurality of n cells in series adjacent connections, eachcell having a positive terminal and a negative terminal; and acapacitor, the method comprises the step of connecting the capacitor inparallel with one of the cells, and repeatedly performing the step ofconnecting the capacitor in parallel with a different one of the cellsthan the previously connected cell.

[0053] One skilled in the art will have no difficulty identifying atleast some of the advantages and disadvantages of the approaches ofsystems 12 and 13 (FIGS. 2 and 3). System 12 has more capacitors thansystem 13, which may take up more room and weigh more. (Most of thedescribed components may be provided in an ASIC or other integratedcircuit, and the only external component or components are the capacitoror capacitors.) On the other hand, system 12 (for a particular size ofcapacitor) has greater bandwidth for equalizing charges since at anygiven instant as many as n-1 current flows may be taking place whichtend to equalize charges, while system 13 has at most one current flowtaking place at a time.

[0054] Interestingly, the two approaches discussed here (systems 12 and13) require fundamentally the same number of bidirectional switchingelements, namely (n-1) times 2.

[0055] An overall comparison of the systems according to the invention,as compared with prior-art systems such as that of FIG. 1, shows severaladvantages.

[0056] First, the dissipative system wastes energy as heat in theresistors. In contrast, the system according to the invention can havearbitrarily small heat losses by minimizing system resistances.

[0057] Second, the systems according to the invention do not requiredetailed or sophisticated control. Instead, the charge transfer activitysimply stops by itself if and when the cells come to be in balance witheach other. These systems are energy-efficient and generate very littleheat. System 13 has a very low external-component count (one capacitor).

[0058] It is possible to identify other possible advantages.Historically, battery makers find it necessary to try very hard to matchcells in a battery, given the “race to the bottom” when one cell failsbefore the other due to performance that differs from that of the othercells. This cell matching costs money because it forces attention tomanufacturing tolerances and the like. With the system according to theinvention, however, it may be possible for the total cost of the batteryto be reduced, even including the charge balancing circuitry, becausethe battery maker may find that it is not so critical to match the cellsto each other.

[0059] Those skilled in the art will appreciate that the precise numberof cells, in this case four, is not important to an understanding of theinvention, and that indeed the invention offers its benefits regardlessof the particular number of cells (so long as it is more than one).Those skilled in the art will also appreciate that while terminal 20 isshown as “positive” and terminal 21 is shown as “ground,” these choicesare arbitrary. The invention offers its benefits equally well to apositive-ground system for example. Finally, it should be appreciatedthat while the invention is described in the context of lead-acid,nickel-cadmium, nickel-metal-hydride, and lithium-ion technologies, theinvention actually offers its benefits for any rechargeableelectrochemical cell used in a battery.

[0060] Those skilled in the art will have no difficulty devising myriadobvious variations and improvements upon the invention without departingfrom the invention in any way, all of which are intended to beencompassed by the claims that follow.

1. Apparatus comprising: a battery comprising a plurality of nrechargeable cells in series-adjacent connections, each cell having apositive terminal and a negative terminal; a plurality of switches, eachcorresponding with one of the cells, each switch having first, second,and third terminals, each switch switchable between a first position inwhich its first and third terminals are connected therebetween and asecond position in which its second and third terminals are connectedtherebetween, the first terminal of each switch connected with thepositive terminal of the corresponding cell, the second terminal of eachswitch connected with the negative terminal of the corresponding cell; aplurality of n-1 capacitors, each connected between third terminals ofadjacent switches; and control circuitry causing all of said switches tobe switched to said first position, and causing all of said switches tobe switched to said second position.
 2. A method for use with a batterycomprising a plurality of n cells in series adjacent connections, eachcell having a positive terminal and a negative terminal; a plurality ofswitches, each corresponding with one of the cells, each switch havingfirst, second, and third terminals, each switch switchable between afirst position in which its first and third terminals are connectedtherebetween and a second position in which its second and thirdterminals are connected therebetween, the first terminal of each switchconnected with the positive terminal of the corresponding cell, thesecond terminal of each switch connected with the negative terminal ofthe corresponding cell; a plurality of n-1 capacitors, each connectedbetween third terminals of adjacent switches, the method comprising:repeatedly performing the steps in sequence of: causing all of saidswitches to be switched to said first position; and causing all of saidswitches to be switched to said second position.
 3. A method for usewith a battery comprising a plurality of n cells in series adjacentconnections, each cell having a positive terminal and a negativeterminal; and a plurality of n-1 capacitors in series adjacentconnections, the method comprising: repeatedly performing the steps insequence of: connecting the capacitors each in parallel with respectiveones of the first through n-1th cells; and connecting the capacitorseach in parallel with respective ones fo the second through n-th cells.4. Apparatus comprising: a battery comprising a plurality of nrechargeable cells in series adjacent connections, each cell having apositive terminal and a negative terminal; first and second multiplexerseach having a plurality of n first terminals and having a secondterminal, the first terminals of the first multiplexer connected withthe positive terminals of the cells, the first terminals of the secondmultiplexer connected with the negative terminals of the cells, eachmultiplexer switchable between n positions in which an n-th firstterminal is connected with the second terminal, a capacitor connectedbetween second terminals of the first and second multiplexers; andcontrol circuitry causing the first and second multiplexers to beswitched to corresponding positions among the n positions.
 5. A methodfor use with a battery comprising a plurality of n rechargeable cells inseries adjacent connections, each cell having a positive terminal and anegative terminal; first and second multiplexers each having a pluralityof n first terminals and having a second terminal, the first terminalsof the first multiplexer connected with the positive terminals of thecells, the first terminals of the second multiplexer connected with thenegative terminals of the cells, each multiplexer switchable between npositions in which an n-th first terminal is connected with the secondterminal, and a capacitor connected between second terminals of thefirst and second multiplexers; the method comprising: repeatedlyperforming the step of causing the first and second multiplexers toswitch both to a corresponding new position.
 6. A method for use with abattery comprising a plurality of n cells in series adjacentconnections, each cell having a positive terminal and a negativeterminal; and a capacitor, the method comprising the step of connectingthe capacitor in parallel with one of the cells, and repeatedlyperforming the step of: connecting the capacitor in parallel with adifferent one of the cells than the previously connected cell.